1. Field of the Invention
This invention relates to a helicoid mechanism of a lens barrel for an optical apparatus such as a single-lens reflex camera or a video television camera (VTR) and more particularly to a helicoid threaded mechanism which consists of male and female helically threaded tubes and satisfies a requirement for high engagement precision and smooth rotation without producing any noise during the sliding engagement of these tube members.
2. Description of the Prior Art
Heretofore, the above-stated male and female helically threaded tube members have been made of a metal material such as aluminum in general. Recently, however, plastic materials have come to be employed in place of the metal material as a result of efforts to reduce the weight and cost of the lens barrel. However, compared with the aluminum material available these days, the plastic materials is somewhat inferior in terms of strength and precision. In view of this, it is now the practice to make the plastic material equivalent to the metal material by using glass fiber or the like in combination with a plastic material. In that instance, however, the glass fiber which is mixed in the plastic material inevitably comes to surface. The tube members which are made of such a material either produce a sliding friction noise between their threaded coupled surfaces or produce there a play resulting from mutual abrasion.
To absorb the play thus produced at the threaded engagement part, it has been contrived to insert an elastic member into the helicoid barrel part in a manner, for example, as disclosed in Japanese Laid-Open Patent Application No. SHO 55-120007. However, in cases where the length of the threaded fitting engagement is short or where the threaded mechanism is required to have a fine lead, the above-stated method of inserting an elastic member fails to meet the required precision. Particularly, the problem of the sliding friction noise produced during the threading rotation still remains unsolved.
In another known method for eliminating the play in engagement and the sliding friction noise of a threaded mechanism, a portion of the threaded part is arranged to differ in the pitch of screw thread or lead from that of other portions as disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 56-9710. However, this arrangement to partly vary the screw thread or lead presents problems such as a difficulty in the manufacture of the threaded tubes.